The Z4 is one of my favorite BMWs for several reasons. 
1) As far as BMWs go, the Z4 is as reliable as they get. I have not had a single major issue with my Z4 and we have owned it for six years. 
Something else I really like about Z4 reliability is that the convertible top has yet to have a single problem. I have owned several other convertible BMWs and this is the only one not to have a serious problem.
2) The handling is absolutely superb. BMWs are known for their superior handling, and it's safe to say that the Z4 is one of the best in the bunch.
3) You can find lots of used Z4s in an affordable price range right now. These vehicles can be bought at a real bargain.